Wireless LAN (WLAN) systems providing broadband wireless access have experienced a spectacular rise in popularity in recent years. While the principal application of these systems has been in providing network connectivity to portable and mobile devices running data applications such as, for example, email and web browsing, there has been a tremendous and growing interest in supporting isochronous services such as telephony service and streaming video.
One of the key issues facing wireless system designers when considering voice and other time-sensitive services over a WLAN connection, such as one described by the IEEE 802.11 specification, is the power consumption of handheld devices. For example, in order to deliver competitive talk time and standby time, as compared to digital cordless or cellular devices, power conservation during voice calls become necessary. Several organizations have proposed power-efficient operation via transmit power control and physical layer rate adaptation for systems that rely on a centrally controlled contention-free channel access scheme. However, such approaches can be complex to implement and may not provide the power savings required to justify the complexity.
The 802.11 standard defines procedures which can be used to implement power management in a handheld device during periods of inactivity. In particular, three distinct building blocks are provided to support power savings: a Wakeup Procedure, a Sleep Procedure, and a Power-save Poll (PS-Poll) Procedure. A mobile client voice station (mobile station) can combine these building blocks in various manners to support power management for different applications.
Wakeup Procedure: There are generally two reasons for the mobile station to wake up, namely to transmit pending data or to retrieve buffered data from the fixed station serving the mobile station, known as an access point. Waking up to transmit data is a straightforward operation, driven by the mobile station. The decision to wake up and receive data is also made by the mobile station after monitoring its pending data bit in a periodic beacon frame transmitted by its access point. Once the mobile station decides to transition from sleep mode to active mode, it notifies the access point by sending an uplink frame with the power-save (PS) bit set to active. Following such transmission, the mobile station remains active so the access point can send any buffered downlink frames afterward.
Sleep Procedure: Similar to the wakeup procedure, a mobile station in the active mode needs to complete a successful mobile station-initiated frame exchange sequence with PS bit set to sleep to transition into the sleep mode. Following this frame exchange sequence, the access point buffers all the downlink frames to this mobile station.
PS-Poll Procedure: Instead of waiting for the access point to transmit the buffered downlink frames, a power-save mobile station can solicit an immediate delivery from its access point by using a PS-Poll frame. Upon receiving this PS-Poll, the access point can immediately send one buffered downlink frame (immediate data response) or simply send an acknowledgement message and response with a data frame later (delayed data response). For the immediate data response case, a mobile station can stay in sleep state after finishing this frame exchange since there is no need for the mobile station to transition to active state given that the access point can only send a buffered downlink frame after receiving a PS-poll from the mobile station. On the other hand, for the delayed data response case, the mobile station has to transition to the active state until receiving a downlink frame from the access point.
The architecture of a simple enterprise WLAN system is depicted in FIG. 1. Referring now to FIG. 1, there is shown a block system diagram overview 100 of a typical enterprise WLAN system. It includes an infrastructure access network 101, consisting of an Access Point 102 and mobile stations such as a data stations 104 and a voice station 106. The mobile stations are connected to the access point via a WLAN radio link 108. The access point is wired to a distribution network, including voice and data gateways 110, 112 respectively, through a switch 114. The voice station runs a Voice-over-IP (VoIP) application, which establishes a peer-to-peer connection with the voice gateway, representing the other end of the voice call, and which routes voice data to a voice network 116. Data stations may connect to the data gateway via the access network and connect to, for example, a wide area network 118. The impact of data traffic on voice quality should be considered. It is assumed that both the voice and data stations employ a prioritized contention-based quality of service mechanism.
VoIP traffic characteristics make voice over WLAN applications uniquely suited for power save operation. In particular, VoIP applications periodically generate voice frames, where the inter-arrival time between frames depends upon the voice coder chosen for an application. The process of encapsulating voice frames into IP packets is commonly referred to as packetization, which is often assumed to occur once every 20 millisecond. A typical VoIP conversation involves a bi-directional constant bit rate flow of VoIP frames, including an uplink flow from the handset to a voice gateway and a downlink flow in the reverse direction.
Since the station generally knows in advance the frame arrival rate, delay, and bandwidth requirements of its voice application, it can reserve resources and set up power management for its voice flows in agreement with the access point. A mobile station may forgo power save mode, and remain in active mode, always ready for the downlink voice transmission. In this case, the access point may transmit downlink voice frames as they arrive. However, if power save is desired, the mobile station may employ the power save building blocks described previously to wake up, exchange the VoIP frame with its access point, and go back to sleep.
In a shared-medium network, such as the access network shown in FIG. 1, it is important to prioritize VoIP traffic over traffic requiring only best-effort delivery, such as the traffic generated by application that can adapt to the amount of bandwidth available in the network and do not request or require a minimum throughput or delay. Prioritization allows the system to minimize the delay experienced by delay-sensitive traffic. A contention-based channel access scheme offering prioritized access named Enhanced Distributed Channel Access (EDCA) has been specified in the IEEE 802.11e draft, and is suitable for VoIP applications. It is based upon the Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA) mechanism defined in 802.11. Stations with voice frames to send must first sense the channel for activity, before transmitting. If the channel has been idle for at least a specified period of time, called an arbitration inter-frame space (AIFS), the mobile station can immediately begin its transmission. Otherwise, the mobile station backs off and waits for the channel to be idle for a random amount of time, which is equal to an AIFS period plus a uniformly distributed value between zero and a contention window (CW) time period value. The CW is further bounded by Minimum contention window (CWmin) and Maximum contention window (CWmax). EDCA provides prioritized access control by adjusting contention parameters: AIFS, CWmin, and CWmax. By selecting different values of AIFS, CWmin, and CWmax for different access categories, the priority to access the medium can be regulated and differentiated. In general, small AIFS, CWmin, and CWmax values result in higher access priority.
It is possible for a mobile station to use information such as the inter-arrival time of downlink voice frames, along with a power-save mechanism, to put itself to sleep between two consecutive voice frames. Presently there are power save procedures described in various papers and WLAN related specifications.
The first prior art power management mechanism utilizes a bit in the packet header. The bit is designated as a power management (PM) bit to signal the change of the power state of the mobile station to the access point. First, a mobile station transitions from sleep mode to active mode upon having an uplink data frame to transmit by setting the PS bit to active in an uplink voice frame to notify the change of its power state. Knowing that there will be one corresponding downlink frame buffered at the access point, because uplink and downlink vocoder share the same voice frame duration, the mobile station stays in active mode for the downlink transmission. After receiving the uplink transmission, the access point then sends buffered downlink frames to the mobile station. In the last downlink frame, the access point sets the “more data” bit to FALSE to communicate the end of the downlink transmission. Finally, the mobile station needs to complete a successful station-initiated frame exchange sequence with PS bit set to sleep to transition into the sleep mode. (e.g. an uplink frame, or a Null frame if there is no uplink data frame to transmit, with the PS bit set to sleep). In the following context, the PS-bit based mechanism is referred to as LGCY6 in the art.
A second power management mechanism uses a PS-Poll frame to solicit downlink frames. Instead of waiting indefinitely for the access point to deliver downlink transmission, the PS-Poll based mechanism utilizes the PS-Poll frame to retrieve the buffered downlink frame from the access point. First, a mobile station transitions to active mode upon having an uplink data frame to transmit. The mobile station then sends out the uplink transmission. Similar to the PS-bit based mechanism, the access point sets the more data field to indicate the presence of any buffered downlink transmission. If the more data bit is TRUE, the mobile station will continue to send a PS-Poll frame to retrieve the buffered downlink frame. Unlike the PS-bit based mechanism, a mobile station can stay in the sleep state since the access point responds to the PS-Poll with an immediate data frame. In the following context, the PS-Poll based mechanism is referred to as LGCY5 in the art.
There are a couple of issues in supporting power-efficient VoIP operation using the current WLAN power save mechanisms. First, the PS-bit based mechanism is somewhat inefficient because, for example, the 802.11 standard currently only offers one way for the mobile station to transition to sleep mode, which is by initiating a frame exchange sequence with PS bit set to sleep. As a result, an extra mobile station initiated frame exchange is needed per bi-directional voice transfer in order for the mobile station to signal power state transition. Since the payload of a voice frame is small (e.g. 20 bytes for voice application with 20 ms framing and 8 Kbps vocoder), the overhead incurred by the extra frame exchange could be as high as one third of the traffic between the mobile station and access point. The significant overhead results in the inefficiency on both power consumption and system capacity.
A second issue is related to quality of service. Under the PS-Poll based mechanism, since a mobile station is not aware of the priority of the buffered downlink frame, the PS-Poll frame is sent as a the best effort access attempt, which is a data traffic mode instead of a voice traffic mode. As a result, the downlink voice transmissions essentially use the best-effort priority instead of the higher voice priority. When a system is loaded with both data traffic using best-effort priority with voice traffic, and a mobile station retrieves downlink voice traffic using a power save poll frame transmitted at the same priority as data traffic, the system will be unable to protect the voice traffic from the delays associated with a congested best-effort delivery system. Legacy power save methods may also require an uplink or poll frame to retrieve each buffered frame for the down link, or require immediate response from the access point for a given uplink frame. One method of providing a particular quality of service is to use scheduled service periods at regular intervals for a given mobile station. This scheduled mode of power save deliver is referred to as automatic power save delivery (APSD). The mobile station wakes up at regular intervals and listens to the channel. The access point is synchronized to the service period, and transmits data at the scheduled time. Thus, the mobile station can put the WLAN subsystem to sleep during the periods between scheduled service intervals. However, this method limits the flexibility of the WLAN channel since there is no ability for the mobile station to deviate from the schedule. Therefore, given these shortcomings of the prior art, there is a need for a reliable power management protocol in a WLAN system that permits mobile station with active voice sessions to efficiently enter and exit power save mode without excessive overhead and maintain quality of service in the presence of lower priority traffic.